This invention relates to a method of generating a tone signal capable of generating a tone signal having desired tone color characteristics by performing a predetermined tone synthesis modulation operation using, as at least one of a modulated signal and a modulating signal, a complex waveshape signal, e.g., a waveshape signal in which the waveshape is caused to change over plural periods by sampling a tone introduced from outside and, more particularly, to effective prevention of an aliasing noise occurring in such tone signal generation method.
U.S. Pat. No. 4,383,462 discloses generation of a tone signal of high quality closely simulating a tone produced by a natural musical instrument by storing a complete waveshape from start of sounding of a tone to the end thereof or a waveshape of plural periods of a part of the complete waveshape and subsequently reading out the stored waveshape from the memory.
Japanese Preliminary Patent Publication No. 29895/1986 or No. 39097/1986 discloses generation of a tone signal of high quality having desired tone color characteristics by performing a tone synthesis modulation operation, e.g., frequency modulation (hereinafter referred to as "FM") or amplitude modulation (hereinafter referred to as "AM") using a waveshape signal of high quality consisting of a waveshape of plural periods stored in a memory such as one described above. According to this method, it is unnecessary to provide, for each different tone color, a memory of a relatively large memory capacity storing a high quality waveshape consisting of a waveshape of plural periods so that high quality waveshapes can be realized in various tone colors with a single waveshape of high quality stored in a memory.
The above described high quality waveshape consisting of a waveshape of plural periods contains complex harmonic components including harmonic components of high frequencies. In a normal tone signal sampling frequency (e.g., about 50 kHz), therefore, an aliasing noise which is inharmonic with the tone pitch of the tone occurs even in a relatively low frequency region and elimination of this aliasing noise has proved very difficult. It is conceivable, for example, to eliminate aliasing noise in a desired frequency region by employing a filter capable of eliminating the aliasing noise. Since, however, aliasing occurs even in a relatively low frequency region with an aliasing point being at a frequency which is one half of the sampling frequency in the case of the frequency components ranging over a relatively high frequency region contained in a tone signal derived by performing modulation operation using a high quality waveshape signal consisting of a waveshape of plural periods and containing complex harmonic components as described above, setting of the cut-off frequency of the filter at a low frequency for eliminating aliasing noise in such low frequency region results in elimination of a part of proper tone signal component. The mere provision of the noise eliminating filter, therefore, does not provide a solution to the problem of aliasing noise.
For the reason stated above, although it has been known in the art that a tone signal of high quality having desired tone color characteristics can be generated with a simple construction, it has been difficult to put it to practice due to the above described problem of aliasing noise.
It is therefore an object of the present invention to provide, in the case of generating a high quality tone signal having desired tone color characteristics with a simple construction by performing tone synthesis modulation operation such as FM and AM using a high quality waveshape signal consisting of a waveshape of plural periods, a method of generating a tone signal capable of eliminating the aliasing noise and thereby enabling practical use of such advantageous tone generation technique.